Early rate of HIV-1 RNA decline in CSF is proportional to the decline in plasma in response to transient administration of single-drug therapy w/either nelfinavir or abacavir. SPECIFIC AIMS: 1) To determine whether initiation of nelfinavir in antiretroviral naive adults is associated w/rapid HIV-1 RNA declines in both plasma and CSF during the first 4 days of therapy. 2) To determine whether initiation of abacavir in antiretroviral naive adults is associated w/rapid HIV-1 RNA declines in both plasma and CSF during the first 4 days of therapy. 3) To determine whether there are differences in CSF HIV-1 RNA decay in response to abacavir vs. nelfinavir, controlling for rates of plasma HIV-1RNA decay. 4)To assess interpatient variability in early virologic response to abacavir and nelfinavir in CSF as compared to plasma.